Through researches on people's daily action habits of using a flashlight, it has been found that people's hands holding the flashlight will generate a downward action when the flashlight is used to illuminate a near distance, because quite bright light is not required at this time; however, people's hands holding the flashlight will generate an upward action when the flashlight is used to illuminate a far distance, so that it is indicated that highest brightness is required at this time. As the brightness of an prior art flashlight is fixed or its brightness adjustment will not be automatically changed according to environmental needs, the relay ability of the flashlight is low and the service life is even reduced due to the use in high brightness.